Joule-Thomson refrigeration systems are available today in several basic designs or configurations. One common configuration includes helically finned tubes coiled around a mandrel. One end of the tubing opens through a restricting orifice into a reservoir for containing the liquified coolant in thermal contact with an object to be cooled, such as an infrared detector. Upon evaporating as it cools the object, low-pressure gas coolant flows in a return path along the fins outside of the tubing to precool the high pressure supply gas flowing inside the tubes toward the reservoir. A more compact configuration winds steel tubing around the outside of a body of a porous matrix material. The matrix is typically made up of many layers of fine wire mesh screen. Low-pressure return gas moving through the central matrix precools the high-pressure gas flowing in the tubing toward the restricting orifice. The configuration is thus one of a short round cylinder, about 2 cm long and 1 cm in diameter. Many variations of these configurations have been developed to improve the heat exchange between the supply and return gas and between the reservoir and the object being cooled, as well as to provide a thermostat controlled supply gas flow regulator for conserving the limited supply of coolant. Examples of refrigeration systems employing these compact configurations are described in U.S. Pat. Nos. 3,942,010 to Peterson et al., 4,468,935 to Albagnac, 4,606,201 and 4,643,001 to Longsworth et al., 4,781,033 to Steyert et al., 5,077,979 to Skertic et al., 5,150,579 to Hingst, 5,382,797 to Kunimoto et al., and 5,111,667 to Hafner et al. Several of these systems employ multiple stages in cascade to reach temperatures of 80-120 K. or less.
Microminiature refrigerator systems using a planar construction have been devised by William A. Little. These planar systems are described by W. A. Little in the article "Microminiature refrigeration, " in Review of Scientific Instruments, vol. 55, no. 5, May 1984, pages 661-680, and in U.S. Pat. Nos. 4,386,505 and 4,392,362. The planar Joule-Thomson refrigerators comprise two or more plates of low thermal conductivity material bonded pressure tight and containing micro-sized gas supply and return passages, such as etched capillary channels at one or more plate interfaces, leading to and from a reservoir or chamber. The heat exchange portion of the refrigerator uses counterflow cooling of the supply gas by the vapor returning from the reservoir, with serpentine microchannels to maximize heat transfer over a short distance. One end of the heat exchange portion is at the ambient temperature, while the other end adjacent to the reservoir is near 80 K. This produces a temperature gradient of about 220 K. over about 5 cm, forcing the use of low thermal conductivity plates to minimize parasitic conduction of heat into the reservoir section. Any heat transfer from the higher temperature inlet end across the plate to the cold reservoir end will limit the refrigeration capacity of the device. Thus, most of the units currently under production by MMR Technology (founded by William Little) are fabricated using glass to provide the needed thermal isolation. Even lower thermal conductivity material is required if the overall size of the device is to be reduced further. Little also discloses several multistage embodiments formed into a stack that employs cascade cooling of one stage by another. In these multilayer devices, the low pressure first gas in one layer is in heat exchange relation with the high pressure second gas in another layer. However, even in these multistage designs, the temperature resistance within layers must remain high. While the use of low thermal conductivity material succeeds in thermally isolating the cold reservoir section in each stage from the warmer heat exchange section of that same stage, it has a disadvantage of also reducing the heat transfer needed to precool the incoming gas with the cold exhaust gas, thus necessitating a larger heat exchanger section. Further, the thermal resistance for heat conduction through the plate material from the gas supply passages to the gas return passages can be significant compared to the convection heat transfer between the gas and the passage walls, making it difficult to optimize the heat exchanger design.
An object of the present invention is to provide a planar-type Joule-Thomson cryostat with rapid cooling capabilities while minimizing both parasitic thermal loads and cryostat volume.